A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, outer pinned layer, coupling layer, inner pinned layer, barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the inner pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer that is magnetized in the same direction. Outer and inner pinned layers are magnetically coupled through a coupling layer that is typically Ru. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons. A TMR sensor is currently the most promising candidate for replacing a giant magnetoresistive (GMR) sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantages of a TMR sensor are a higher MR ratio and the preference for CPP geometry for high recording density. A high performance TMR sensor requires a low areal resistance RA (area×resistance) value, high MR ratio, a soft free layer with low magnetostriction (λ), a strong pinned layer, and low interlayer coupling through the barrier layer. The MR ratio is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2. As a consequence, MR ratio drops significantly. To maintain a reasonable signal-to-noise (SNR) ratio, a novel magnetic tunneling junction (MTJ) with a lower RA value and higher MR ratio higher than provided by conventional MTJs is desirable.
A MgO based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for AlOx or TiOx based MTJs. A very high MR ratio has been reported by Yuasa et al. in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004), and by Parkin et al. in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867 (2004). Yuasa and Parkin demonstrated that an MR ratio of ˜200% can be achieved at room temperature in epitaxial Fe(001)/MgO(001)/Fe(001) and polycrystalline FeCo(001)/MgO(001)/(Fe70Co30)80B20 MTJs. Yuasa et al. also report a very high TMR ratio in “Giant tunneling magnetoresistance up to 410% at room temperature in fully epitaxial Co/MgO/Co magnetic tunnel junctions with bcc Co(001) electrodes”, Applied Physics Letters 89, 042505 (2006). Meanwhile, Djayaprawira et al. in “230% room temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, Appl. Physics Letters 86, 092502 (2005) showed that MTJs having a CoFeB/MgO(001)/CoFeB structure made by conventional sputtering can also have a very high MR ratio with the advantages of better feasibility and uniformity. However, RA values in the MTJs mentioned above are in the range of 240 to 10000 ohm-um2 which is too high for read head applications. To solve this issue, Tsunekawa et al. in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005) found a reduction in RA by inserting a DC-sputtered metallic Mg layer between a bottom CoFeB layer and rf-sputtered MgO. The Mg layer improves the crystal orientation of the MgO(001) layer when the MgO(001) layer is thin. The MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm-uM2. The idea of metallic Mg insertion was initially disclosed by Linn in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure. Alternatively, a Ta getter pre-sputtering prior to the rf-sputtering of the MgO layer can also achieve 55% TMR ratio with low RA as recently reported by Y. Nagamine et al. in “Ultralow resistance-area product of 0.4 ohm-um2 and high magnetoresistance above 50% in CoFeB/MgO/CoFeB magnetic junctions”, AppI. Physics Letters 89, 162507 (2006).
Although a high MR ratio and low RA have been demonstrated in MTJs having a MgO barrier layer, there are still many issues to be resolved before such configurations can be implemented in a TMR sensor of a read head. For example, the annealing temperature needs to be lower than 300° C. for read head processing, and rf-sputtered MgO barriers make control of RA mean and uniformity more difficult than with conventional DC-sputtered and subsequently naturally oxidized AlOx barriers. Moreover, a CoFe/NiFe free layer is preferred over CoFeB for low λ and soft magnetic properties but when using a CoFe/NiFe free layer in combination with a MgO barrier, the MR ratio will degrade to very near that of a conventional AlOx MTJ. Thus, a TMR sensor is needed that incorporates a MgO barrier without compromising any desirable properties such as high MR ratio, a low RA value, and low magnetostriction.
In other prior art, a free layer having a trilayer CoFe/CoFeB/NiFe configuration is disclosed in U.S. Patent Application 2007/0047159.
U.S. Pat. No. 6,493,196 teaches a pinned layer comprising CoFeB/Ru/CoFeB or CoFe/Ru/CoFeB structure. In U.S. Pat. No. 6,995,960, a CoFeB/Ru/CoFeB pinned layer is disclosed.
U.S. Pat. No. 7,161,774 teaches the use of alloys and amorphous materials such as NiFe, Co, CoFe, NiFeCo, FeCo, CoFeB, CoZrMo, CoZrNb, CoZr, CoZrTa, CoTaHf, CoNbHf, CoHfPd, CoTaZrNb, and CoZrMoNi for forming a fixed layer. Composite pinned layers (AP2/coupling layer/AP1) including Co/Ru/Co, CoFe/Ru/CoFe, and CoFeNi/Ru/CoFeNi are described but there is no suggestion of a composite AP1 layer.
U.S. Pat. No. 7,163,755 discloses that a pinned layer may include CoFeB, a Fe-based material, or a material containing at least 50% by weight of Fe, Co, or Ni.